I have 15x7 with 4.25 back space now , will 15x8 with 4.625 BS fit in back

Be safe and just measure it. Go to any wheel manufacturer website get their work sheet to determining how to measure it. Its actually easy to do and the safest way to determine since all of our cars vary slightly from car to car. Copy and past this link it will give you the measuring you need to determine specific for your car.

http://www.billetspecialties.com/wheels/downloads/wheel-measure-guide/